Endogenous angiotensin within the rostral ventrolateral medulla facilitates the somatosympathetic reflex

Objective: To determine whether endogenous angiotensin modifies the synaptic excitation of sympatho-excitatory neurons in the rostral part of the ventrolateral medulla. Design and methods: Experiments were performed on anaesthetized rabbits with denervated arterial and cardiopulmonary baroreceptors. Arterial pressure, heart rate and renal sympathetic nerve activity were measured. The average sympatho-excitatory reflex response evoked by short-train stimulation of the sciatic nerve was measured before and at various times after micro-injection of the non-specific angiotensin receptor antagonist [Sar¹, Thr⁸]-angiotensin II (80pmol) into the contralateral rostral ventrolateral medulla. Because the central pathway mediating this somatosympathetic reflex includes a synapse within the contralateral (but not ipsilateral) rostral ventrolateral medulla, a change in the evoked response after blockade of angiotensin receptors in the contralateral rostral ventrolateral medulla indicates a role of these receptors in synaptic transmission within this region. As a control, the effects on this reflex of micro-injection of [Sar¹, Thr⁸]-angiotensin II into the ipsilateral rostral ventrolateral medulla were also measured. Results: After injection of [Sar¹Thr⁸-angiotensin II into the contralateral rostral ventrolateral medulla, the evoked sympatho-excitatory reflex response was significantly reduced compared with the response before injection. In contrast, injection of [Sar¹Thr⁸-angiotensin II into the ipsilateral rostral ventrolateral medulla had no significant effect on the evoked sympatho-excitatory reflex response, although contralateral and ipsilateral injections of [Sar¹Thr⁸-angiotensin II had very similar effects on baseline arterial pressure and renal sympathetic nerve activity. Conclusions: The results indicate that, in the anaesthetized rabbit with denervated arterial and cardiopulmonary baroreceptors, endogenous angiotensin is tonically released within the rostral ventrolateral medulla and facilitates the synaptic excitation of sympatho-excitatory neurons reflexly evoked by stimulation of afferent fibres in the sciatic nerve.